The invention relates to a transmission between an electric motor and a tool shaft, for instance for hand tools such as an electric screwdriver and the like, which transmission is provided with an adjustable breaking coupling for discontinuing the drive torque on the tool shaft when a predetermined resistance moment on this tool shaft is exceeded.
In electric tools, particularly electric hand tools, it is known to place a slip or claw coupling between the electric motor and the tool shaft, whereby in the case of overload the tool shaft is no longer subjected to the full torque of the electric motor. The drawback to such a system is that when the motor is driven, a torque is still exerted continuously or intermittently on the tool shaft. This can be disadvantageous in particular applications. In addition, such couplings are noisy and greatly subject to wear.
There also exist protection circuits which cause the motor feed to be switched off and/or braked as soon as overload of the motor occurs. Such a switch-off system is difficult to embody particularly in conjunction with battery-powered DC-motors because the high amperages could present adverse consequences during switch-off upon overload. Moreover, the mass inertia of the rotating parts continues to act on the tool shaft during switch-off.
The object of the invention is to provide a transmission wherein a disengagement takes place between motor and tool shaft immediately after the desired resistance moment is exceeded, wherein the inertia of the rotating parts no longer has any effect on the tool shaft so that it stops immediately.